Selecting the right medium voltage switchgear lineup is one of the most consequential decisions in any power distribution project. Get it wrong, and you face years of costly maintenance headaches, compliance failures, or — worst of all — arc flash incidents. Get it right, and your lineup delivers 20–30 years of reliable, low-intervention service.

This guide cuts through the noise with a fast, structured decision tree specifically built for the 6kV, 10kV, and 35kV voltage tiers — the three bands that cover the vast majority of industrial and utility MV applications worldwide. Whether you are specifying a new substation, upgrading aging equipment, or evaluating a high and low voltage switchgear lineup for an industrial facility, this framework will help you reach a confident selection in minutes, not weeks.

What Is MV Switchgear and Why the 6–35kV Range Matters

Under IEC 62271, medium voltage (MV) switchgear covers electrical assemblies rated between 1kV and 35kV. These systems perform four critical functions in any power network: switching, fault interruption, isolation for maintenance, and protection coordination. In North America, ANSI/IEEE standards sometimes extend the MV definition up to 69kV, but the 6–35kV band remains the engineering sweet spot for most industrial and distribution applications globally.

The three voltage tiers within this band serve distinct roles:

• 6kV systems are common in Eastern European, Russian, and some Asian industrial grids, particularly in legacy chemical plants, mining operations, and motor drive applications where 6kV motors are already installed.
• 10kV (or 11kV) systems represent the most widely deployed MV tier globally. Urban distribution networks, manufacturing facilities, data centers, and commercial buildings overwhelmingly use this voltage level.
• 35kV (or 33kV) systems appear at the boundary between distribution and sub-transmission. Wind farms, large industrial complexes, and primary substations feeding downstream 10kV networks typically operate at this level.

Understanding which tier your project falls into is the starting point of every lineup decision — but it is far from the only variable.

The Core Decision Variables Before You Choose a Lineup

Before entering the decision tree, engineers must lock down five parameters. Leaving any one of them undefined almost always leads to an undersized, over-specified, or non-compliant lineup.

Once these five parameters are defined, the decision tree below provides a clear, step-by-step path to the right lineup type.

The Fast Decision Tree: From Voltage to Lineup Type

Work through the following four steps sequentially. Each step eliminates incompatible options and narrows the field until one lineup category emerges as the clear fit.

Step 1 — Lock Your Voltage Tier

Your system nominal voltage determines the rated voltage class of the switchgear. Always specify equipment rated at the next standard level above nominal to provide insulation margin.

• 6kV nominal → specify 7.2kV-rated equipment
• 10kV nominal → specify 12kV-rated equipment
• 35kV nominal → specify 40.5kV-rated equipment

Step 2 — Assess Your Space and Environment

Space and environmental conditions eliminate the majority of unsuitable lineup types early.

• Constrained indoor space, urban substation, or offshore platform → GIS or solid-insulated switchgear (SIS). Their compact footprint can be 40–60% smaller than equivalent AIS lineups.
• Standard indoor installation, controlled environment → AIS metal-clad or metal-enclosed switchgear. Widely available, cost-effective, and easy to specify.
• Outdoor installation → Either NEMA 3R-rated AIS lineups or GIS with appropriate weather-resistant enclosures. For 35kV outdoor applications, GIS becomes strongly preferred.
• Harsh environments (high humidity, coastal salt fog, heavy industrial pollution) → SIS or GIS with stainless steel or aluminum enclosures and enhanced IP ratings (IP54 minimum).

Step 3 — Define Your Maintainability Requirement

How quickly must you restore service after a fault or scheduled maintenance? This determines whether you need a fixed, withdrawable, or fully compartmentalized design.

• Mission-critical facilities (hospitals, data centers, airports) → Withdrawable circuit breaker design. The breaker truck rolls out in minutes without de-energizing adjacent feeders, dramatically reducing mean time to restore (MTTR).
• Industrial plants with planned shutdown windows → Fixed or semi-fixed designs are acceptable. Lower upfront cost offsets the need to schedule outages.
• Utility and ring main applications → Ring Main Units (RMU) with load break switches provide fast manual or automated re-routing without breaker withdrawal.

Step 4 — Choose Your Insulation Medium

Insulation medium is the final filter, and in 2025 it carries a strong environmental dimension.

• Vacuum interrupters (standard for 6–35kV) — The default choice for circuit breaker interrupting elements in MV switchgear. Environmentally clean, long service life (typically 10,000 operations), and maintenance-free interrupter. Recommended for virtually all new 6–35kV projects.
• SF₆ gas insulation — Historically dominant in GIS designs for its superior dielectric strength. However, SF₆ has a Global Warming Potential approximately 23,500 times that of CO₂. Many jurisdictions are restricting or taxing SF₆ equipment. Only specify SF₆ where SF₆-free alternatives are technically unavailable.
• SF₆-free alternatives (dry air, nitrogen, g³ gas) — Now available for most 12kV and selected 40.5kV GIS applications. Specify these where future-proofing against environmental regulation is a priority.
• Solid insulation (epoxy resin, cast resin) — Used in solid-insulated switchgear (SIS) and ring main units. Fully sealed, no gas handling required, excellent for high-humidity environments. Well-proven at 12kV; increasingly available at 40.5kV.

Combining the outputs of all four steps yields the following decision matrix:

AIS vs GIS vs Solid-Insulated: A Quick Comparison

For engineers still weighing insulation technology, the table below distills the key trade-offs across the three mainstream options for 6–35kV lineups.

Common Selection Mistakes Engineers Make

Even experienced engineers fall into predictable traps when specifying MV switchgear lineups. Knowing these pitfalls in advance can save significant time and cost during commissioning or, worse, after a fault event.

Mistake 1: Specifying rated voltage equal to system voltage. A 10kV system requires 12kV-rated equipment — not 10kV-rated. Using an exact-match voltage rating leaves no insulation margin for transient overvoltages and violates IEC 62271-1 recommendations. Always specify the next standard rated voltage above the system nominal.

Mistake 2: Underestimating the prospective short-circuit current. Short-circuit levels at MV buses frequently increase as utilities upgrade transmission capacity upstream. Equipment specified for 16kA may face 20kA fault currents within five years of commissioning. Build in headroom — specify 25kA interrupting capacity when the present fault level is 20kA or below.

Mistake 3: Ignoring internal arc classification (IAC). IEC 62271-200 defines internal arc accessibility categories (A, B, C). Selecting a non-IAC-rated lineup for an installation where personnel regularly work near energized equipment is a compliance failure and a serious safety risk. Always confirm the required IAC class during the design stage, not during procurement.

Mistake 4: Specifying SF₆-insulated GIS without checking local regulations. Several jurisdictions in the EU and North America now restrict or impose CO₂-equivalent taxes on SF₆ equipment. Projects with 10-year or longer service horizons should evaluate SF₆-free alternatives from the outset to avoid costly future retrofits.

Mistake 5: Neglecting future expansion slots. Switchgear lineups are routinely extended five or ten years after initial installation. Failing to reserve bus-end extension provisions — or selecting a non-extensible fixed-bus design — forces costly re-buswork or parallel lineups later. Plan for at least 20% spare panel capacity at the design stage.

How to Specify a 6–35kV Switchgear Lineup: Key Checklist

Once the decision tree has pointed you to the correct lineup type, the following checklist ensures your specification document captures every parameter required for compliant procurement and accurate quoting.

1. Rated voltage class (7.2 / 12 / 17.5 / 24 / 40.5kV)
2. Rated continuous current of busbars and circuit breakers (A)
3. Rated short-circuit breaking current (kA) and duration (1 or 3 seconds)
4. Rated internal arc current and accessibility category (IEC 62271-200)
5. Insulation medium: air / SF₆ / SF₆-free / solid
6. Circuit breaker design: fixed / withdrawable (truck) / plug-in
7. Number of incoming, bus-coupler, feeder, and metering panels
8. Protection relay type and communication protocol (IEC 61850, Modbus, DNP3)
9. CT and PT ratios, accuracy classes, and burden
10. Enclosure type: NEMA 1 indoor / NEMA 3R outdoor / IP54 / IP65
11. Applicable standards: IEC 62271-200, IEC 62271-100, or ANSI/IEEE C37.20
12. Type test reports and third-party certification (ASTA / KEMA / CCC)

For projects combining MV switchgear with step-down transformation, coordinating the switchgear lineup with upstream and downstream equipment is essential. Our distribution transformer range and prefabricated substation solutions are engineered to integrate seamlessly with 6–35kV lineups, simplifying system coordination and reducing project delivery time.

If your project falls within the 6–35kV range — whether a new industrial substation, a renewable energy collector bus, or a utility feeder upgrade — our engineering team is ready to review your single-line diagram and recommend the optimal lineup configuration. Contact us with your system parameters to receive a detailed technical proposal.